This invention relates to ferroelectric, or tilted chiral smectic, liquid crystal display devices.
U.S. Pat. No. 4,367,924 (Clark et al.) discloses an electro-optical device including a chiral smectic C or H liquid crystal disposed between flat plates treated to enforce molecular orientation parallel to the plates. The plates are spaced by a distance sufficiently small to ensure unwinding of the helix typical in a bulk of the material to form two stable states of the orientation field.
U.S. Pat. No. 4,563,059 (Clark et al.) discloses a liquid crystal device including a ferroelectric liquid crystal disposed between plates treated to enforce a particular ferroelectric molecular orientation to the plates. The devices employ along or in combination non-planar boundary conditions, polar boundary conditions, boundaries with multiple physical states, intrinsic spontaneous splay distortion of the polarization orientation field, combined ferroelectric and dielectric torques, layer tilted with respect to the plates. The plates are spaced by a distance sufficiently small to ensure unwinding of the helix typical in a bulk of the material to form either monostable, bistable or multistable states which exhibit novel electro-optic properties.
U.S. Pat. No. 4,861,143 (Yamazaki et al.) discloses a chiral smectic liquid crystal display having grey scale caused by applying a voltage of an intermediate level. Within each picture element of the display are a number of domains of the liquid crystal layer, some being transparent while the other being opaque rendering grey tone to the picture element. In response to an intermediate voltage, the liquid crystal layer resides in an intermediate condition where some molecules favor the transparent condition and other molecules favor the opaque condition within one picture element. The intermediate condition is stabilized and realizes diverse grades of transmissivity by virtue of fluctuation of the distance between the opposed substrates within one picture element, i.e., by virtue of fluctuation of the electric field applied thereon. In other words, a picture element is sectioned into a plurality of domains having different threshold voltages.
Lagerwall, S. T., Wahl, J., and Clark, N. A., "Ferroelectric Liquid Crystals for Displays", 1985 International Display Research Conference, pp. 213-221, describe liquid crystal devices having grey scale capacity generated by multi-domain, fine grained cell surface structure (space integration) or by switching with different duty ratio (time integration).
Ross, P. W., "720.times.400 Matrix Ferroelectric Display Operating at Video Frame Rate", 1988 International Display Research Conference, pp. 185-190, discloses that grey scale can be introduced in a ferroelectric display by two methods. One is to split pixels into smaller addressable regions. Such a method requires fine photolithographic geometries and can also increase the required addressing rate if an increase in the number of scanned lines results. An alternative method is to keep simple pixel structures but use repeated addressing within a frame time to vary the duty cycle of light to dark time of the pixel. Ross also discloses that in practice the optimum grey scale technique is likely to be a combination of these spatial and temporal dither techniques.
Hartmann, W. J. A. M., "Ferroelectric Liquid Crystal Video Display", 1988 International Display Research Conference, pp 191-194, describes using a multi-domain structure to generate grey scale, whereby, defects are artificially induced by surface preparation techniques.
Clark, N. A., et al., "Modulators, Linear Arrays, and Matrix Arrays Using Ferroelectric Liquid Crystals", Proceedings of Society for Information Display, Vol. 26, 1985, pp. 133-139, describes temporal dither to obtain grey scale. This method turns the device pixels on and off for varying amounts of time, resulting in variable transmitted light energy, which is observed as grey scale. This method complicates the driving scheme for the device and slows down the speed of the display update.
Tomikawa, T. "An Identification of Pseudo Continuous Tone by Labeling", Conference Record of the 1988
International Display Research Conference, pp. 146-151, describes spatial dither to obtain grey scale. This method breaks a super pixel into much smaller independently addressable areas. By turning on varying numbers of these smaller elements, grey scale is achieved. This method complicates the device by increasing the number of display elements that need to be addressed.
Leroux, T., et al., "Black and White FLC TV Panel with Grey Levels", 1988 International Display Research Conference, pp. 111-113, discloses a ferroelectric liquid crystal TV panel with grey levels achieved using a combination of multidomain switching, time integration, and space integration. This is a compromise between the spatial method which requires a large number of connectors for the large number of sub-pixels, and the temporal method which requires extremely fast liquid crystal material response times.